1. Field of the Invention
This invention relates generally to the control of oil and gas production wells. More particularly, it relates to control of movable elements in well production flow control devices.
2. Description of the Related Art
The control of oil and gas production wells constitutes an on-going concern of the petroleum industry due, in part, to the enormous monetary expense involved in addition to the risks associated with environmental and safety issues. Production well control has become particularly important and more complex in view of the industry wide recognition that wells having multiple branches (i.e., multilateral wells) will be increasingly important and commonplace. Such multilateral wells include discrete production zones which produce fluid in either common or discrete production tubing. In either case, there is a need for controlling zone production, isolating specific zones and otherwise monitoring each zone in a particular well. Flow control devices such as sliding sleeve valves, downhole safety valves, and downhole chokes are commonly used to control flow between the production tubing and the casing annulus. Such devices are used for zonal isolation, selective production, flow shut-off, commingling production, and transient testing.
It is desirable to operate the downhole flow control device with a variable flow control device. The variable control allows the valve to function in a choking mode which is desirable when attempting to commingle multiple producing zones that operate at different reservoir pressures. This choking prevents crossflow, via the wellbore, between downhole producing zones.
In the case of a hydraulically powered flow control device such as a sliding sleeve valve, the valve experiences several changes over time. For example, hydraulic fluid ages and exhibits reduced lubricity with exposure to high temperature. Scale and other deposits will occur in the interior of the valve. In addition, seals will degrade and wear with time. For a valve to act effectively as a choke, it needs a reasonably fine level of controllability. One difficulty in the accurate positioning of the moveable element in the flow control device is caused by fluid storage capacity of the hydraulic lines. Another difficulty arises from the fact that the pressure needed to initiate motion of the moveable element is different from the pressure needed to sustain motion, which is caused by the difference between static and dynamic friction coefficients, with the static coefficient being larger than the dynamic coefficient. When pressure is continuously applied through the hydraulic line, the elastic nature of the lines allows some expansion that, in effect, causes the line to act as a fluid accumulator. The longer the line the larger this effect. In operation, the combinations of these effects can cause substantial overshoot in the positioning of the moveable element. For example, if the hydraulic line pressure is raised to overcome the static friction, the sleeve starts to move. A known amount of fluid is commonly pumped into the system to move the element a known distance. However, because of the fluid storage effect of the hydraulic line and the lower force required to continue motion, the element continues to move past the desired position. This can result in undesirable flow restrictions.
The present invention overcomes the foregoing disadvantages of the prior art by providing a system and method for overcoming the static friction while substantially reducing the overshoot effect. Still other advantages over the prior art will be apparent to one skilled in the art.